This invention relates to the field of AM receivers for receiving compatible stereo signals and more particularly to the prevention of distortion generated when the received signal is essentially a left or right signal only.
Since a stereo broadcast signal must be receivable without distortion by monophonic receivers, the signal should have only monophonic information on the envelope of the carrier. It must, however, contain sufficient information to enable stereo receivers to decode the broadcast signal and produce the original, undistorted stereo signals. Such a signal, with transmitters and receivers for utilizing it, is described in a co-pending application Ser. No. 674,703, assigned to the same assignee as is the present application. This signal, for broadcasting two independent signals which could be left and right stereophonic signals, is of the form (L+R)cos(.omega..sub.c t+.phi.) where .phi. is arc tan [(L-R)/(1+L+R)]. A correction factor proportional to cos .phi. is used to restore the original difference signal. This correction factor can be produced by coupling an amplitude limited signal to a phase lock loop as described in the above-referenced patent application. If, however, the broadcast signal contains left or right information only for any period of time, the correction factor may include a static phase error. It is desirable to prevent such an error from being introduced into the correction factor as it could produce distortion in the corrected signals.